U.S. patent number 6,954,643 [Application Number 10/606,428] was granted by the patent office on 2005-10-11 for criteria for base station selection, including handover, in a wireless communication system.
This patent grant is currently assigned to ArrayComm LLC. Invention is credited to Paul Petrus.
United States Patent |
6,954,643 |
Petrus |
October 11, 2005 |
Criteria for base station selection, including handover, in a
wireless communication system
Abstract
The present invention provides method and apparatus for
facilitating base station selection/handover by a user terminal in
a distributed (e.g., cellular-type) wireless communication system.
In accordance with one aspect, hysteresis is adaptively determined
as a function of the variance of receive signal strength
fluctuations. In turn, an adaptive hysteresis factor can be
obtained and used for a subsequent handover decision, for example,
based on a cost function that takes into account the hysteresis. In
accordance with another aspect, base station selection depends on a
number of criteria, such as received signal strength, base station
load, and estimated distance between a receiving user terminal and
one or more base stations.
Inventors: |
Petrus; Paul (Santa Clara,
CA) |
Assignee: |
ArrayComm LLC (San Jose,
CA)
|
Family
ID: |
33540053 |
Appl.
No.: |
10/606,428 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
455/437; 342/457;
370/332; 455/436; 455/456.1; 455/524 |
Current CPC
Class: |
H04W
48/20 (20130101); H04B 17/318 (20150115); H04B
17/382 (20150115); H04W 36/08 (20130101); H04W
48/16 (20130101) |
Current International
Class: |
H04B
17/00 (20060101); H04Q 7/38 (20060101); H04Q
007/20 (); H04Q 007/00 (); G01S 003/02 () |
Field of
Search: |
;455/453,435.2,452.1,450,67.11,67.13,67.16,525,524,150.1,403,502,436
;370/332 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trost; William
Assistant Examiner: Doan; Kiet
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor &
Zafman LLP
Claims
What is claimed is:
1. A method for selecting a base station comprising: receiving
transmissions from a plurality of base stations; deriving from the
transmissions indications of received signal strength associated
with each of the plurality of base stations; ordering a set of
candidate base stations in accordance with descending indications
of received signal strength; deriving from the transmissions load
information associated with each of the plurality of base stations;
comparing load information between a likely selected base station
and the remaining set of candidate base stations; deriving from the
transmissions distance information associated with each of the
plurality of base stations; comparing distance information between
the likely selected base station and the remaining set of candidate
base stations; selecting one of the plurality of base stations as a
current base station when the load of and distance to the likely
selected base station does not exceed the load of and distance to
each of the remaining set of candidate base stations by a first
threshold; and selecting an alternative base station when the load
of and distance to the likely selected base station exceeds the
load of and distance to an alternative base station of the
remaining set of candidate base stations by the first
threshold.
2. The method of claim 1, wherein selecting an alternative base
station further comprises selecting an alternative base station
when also a handover cost function (C.sub.j) associated with the
alternative base station exceeds a handover cost function (C.sub.j)
associated with the likely selected base station by a second
threshold.
3. The method of claim 1, wherein deriving distance information
comprises computing distance based on a reference time of
transmission indicated in each of the transmissions and a received
time of each of the transmissions.
4. The method of claim 1, wherein comparing load information
comprises comparing load information using base station pairs in
the order of descending indications of received signal
strength.
5. The method of claim 1, further comprising: eliminating a first
candidate base station among a plurality of candidate base stations
if the load information associated with the first candidate base
station indicates the load of the first candidate base station
exceeds a threshold.
6. The method of claim 1, wherein the first threshold is equal to
two (2).
7. The method of claim 1, wherein the second threshold is equal to
3 dB.
8. The method of claim 1, wherein deriving received signal strength
information associated with transmission from each of the plurality
of base stations comprises computing a cost function based on the
received signal strength of base station pairs of the plurality of
base stations and a hysteresis factor.
9. The method of claim 1, wherein the hysteresis factor is
adaptively determined based on standard deviation of the received
strength for each base station pair.
10. A machine-readable medium having stored thereon a set of
machine-executable instructions that, when executed by a
data-processing system, cause the system to perform a method for
selecting a base station comprising: receiving transmissions from a
plurality of base stations; deriving from the transmissions
indications of received signal strength associated with each of the
plurality of base stations; ordering a set of candidate base
stations in accordance with descending indications of received
signal strength; deriving from the transmissions load information
associated with each of the plurality of base stations; comparing
load information between a likely selected base station and the
remaining set of candidate base stations; deriving from the
transmissions distance information associated with each of the
plurality of base stations; comparing distance information between
the likely selected base station and the remaining set of candidate
base stations; selecting one of the plurality of base stations as a
current base station when the load of and distance to the likely
selected base station does not exceed the load of and distance to
each of a the remaining set of candidate base stations by a first
threshold; and selecting an alternative base station when the load
of and distance to the likely selected base station exceeds the
load of and distance to an alternative base station of the
remaining set of candidate base stations by the first
threshold.
11. The machine-readable medium of claim 10, wherein selecting an
alternative base station further comprises selecting an alternative
base station when also a handover cast function (C.sub.j)
associated with the alternative base station exceeds a handover
cost function (C.sub.j) associated with the likely selected base
station by a second threshold.
12. The machine-readable medium of claim 10, wherein deriving
distance information comprises computing distance based on a
reference time of transmission indicated in each of the
transmissions and a received time of each of the transmissions.
13. The machine-readable medium of claim 10, wherein comparing load
information comprises comparing load information using base station
pairs in the order of descending indications of received signal
strength.
14. The machine-readable medium of claim 10, wherein the method
further comprises: eliminating a first candidate base station among
a plurality of candidate base stations if the load information
associated with the first candidate base station indicates the load
of the first candidate base station exceeds a threshold.
15. The machine-readable medium of claim 10, wherein the first
threshold is equal to two (2).
16. The machine-readable medium of claim 10, wherein the second
threshold is equal to 3 dB.
17. The machine-readable medium of claim 10, wherein deriving
received signal strength information associated with transmission
from each of the plurality of base stations comprises computing a
cost function based on the received signal strength of base station
pairs of the plurality of base stations and a hysteresis
factor.
18. The machine-readable medium of claim 10, wherein the hysteresis
factor is adaptively determined based on standard deviation of the
received strength for each base station pair.
19. A user terminal comprising: a receiver to receive transmissions
from a plurality of base stations; a receive signal strength
measurement unit to derive indications of received signal strength
for each of the transmissions; a distance calculation unit to
derive from the transmissions distance information associated with
each of the plurality of base stations; and a base station
selection unit to: order a set of candidate base stations in
accordance with descending indications of received signal strength,
to derive from the transmissions load information associated with
each of the plurality of base stations; compare load information
between a likely selected base station and the remaining set of
candidate base stations; compare distance information between the
likely selected base station and the remaining set of candidate
base stations; select one of the plurality of base stations as a
current base station when the load of and distance to the likely
selected base station does not exceed the load of and distance to
each of the remaining set of candidate base stations by a first
threshold; and select an alternative base station when the load of
and distance to the likely selected base station exceeds the load
of and distance to an alternative base station of the remaining set
of candidate base stations by the first threshold.
20. The user terminal of claim 19, wherein selecting an alternative
base station further comprises selecting an alternative base
station when also a handover cost function (C.sub.j) associated
with the alternative base station exceeds a handover cost function
(C.sub.j) associated with the likely selected base station by a
second threshold.
21. The user terminal of claim 19, wherein the distance calculation
unit derives distance information by computing distance based on a
reference time of transmission indicated in each of the
transmissions and a received time of each of the transmissions.
22. The user terminal of claim 19, wherein the signal strength
measurement unit derives indications of received signal strength by
computing a cost function based on the received signal strength of
base station pairs of the plurality of base stations and a
hysteresis factor.
Description
FIELD OF THE INVENTION
The present invention relates to the field of wireless
communications systems, and in particular, to a method and system
for selecting a base station.
BACKGROUND OF THE INVENTION
Distributed or "cellular" radio communication systems are typically
comprised of a number of cells, with each cell corresponding
roughly to a geographical area. Each cell has an associated base
station (BS) which is a local central site providing access to the
communication system to a number of radio transmitter/receiver
units (user terminals (UTs)) within the cell.
In such distributed wireless communication systems, a user terminal
generally has the capability to move in and out of various cells.
While a base station typically handles traffic exchange with a
number of user terminals within its cell, a user terminal typically
exchanges traffic with a single base station at a time. However,
when communication quality degrade swith respect to a particular
base station(s) and improves with respect to one or more other base
stations (e.g., due to changes in the RF environment, movement of
the user terminal away/toward cells, etc.), the user terminal
should be able to handover "active communication" to the base
station providing better communication.
FIG. 1 depicts a typical distributed voice and/or data network
topology, in accordance with the prior art. As shown, a number of
base stations 102, 104, and 106 are distributed geographically,
such that each provides a cell coverage area (or simply cell) 112,
114, and 116, respectively. In turn, the cells 112, 114, and 116
each has an associated cell boundary 118, 120, and 122,
respectively, simplistically depicted circularly in FIG. 1.
As a user terminal 108 shown in cell 114 (and therefore assumed to
be in active communication, i.e., exchanging voice and/or data
traffic, with the associated base station 104 which may be coupled
to a larger voice and/or data network, such as PSTN or the
internet) moves in a direction 110 away from the base station 104
and toward the base station 106 and its associated cell boundary
122, handover will typically take place--i.e., the user terminal's
active communication session will transfer to the new base station
106. As such, if the user is, for example, exchanging data with a
device coupled to the internet (which in turn is coupled to the
base stations shown in the network 100 of FIG. 1), such exchange
will not be interrupted due to the user terminal's movement away
from the cell coverage of one station; the exchange or session will
be handed over to another base station.
Handover schemes are typically based on optimizing a cost function
(C) that depends on one or more parameters, such as the received
signal strengths (RSSI) from one or more base stations within the
communication range of the user terminal, or visa versa, at a given
time. As such, the scheme can be implemented by the user terminal,
the base station, or a combination thereof. Most common schemes
involve user terminal-centric handover, i.e., measurements (e.g.,
RSSI) made by the UT are solely used in the handover cost function.
In such schemes, the user terminal periodically samples
transmissions (e.g., the broadcast message(s)) from a set of base
stations, including any with which it is in active communication,
and optimizes the cost function to select the best base
station.
Because of the fluctuations (e.g., due to shadowing and other
real-world RF effects) in the power of each base station as
experienced by a user terminal, the handover cost function C
usually should include a margin, sometimes known as a hysteresis
margin. In other words,
where h is the hysteresis (sometimes also referred to as the
hysteresis margin or factor), S.sub.A is the RSSI of the current
(or also sometimes referred to as the active) base station, and
S.sub.i is the RSSI of an ith candidate base station. When C<0,
then handover should be initiated.
The hysteresis margin h is included in the cost function to prevent
the user terminal from frequent "ping ponging" between two or more
base stations due to the power fluctuations of their transmissions.
This particular need for h is further illustrated and described in
connection with FIG. 2.
FIG. 2 is a graphical representation of the theoretical and
practical signal strengths of two base stations experienced by a
user terminal as the user terminal moves away from one of the base
stations with which it is in active communication and toward the
other base station. The signal strength of each base station's
transmissions as received at the user terminal 108 is represented
by the vertical axis as a function of distance as the user terminal
108 moves with a velocity 110 away from base station 104 and toward
base station 106.
Ignoring the practical power fluctuations described above, the
pathloss associated with the base stations 104 and 106, given the
user terminal 108's described motion, could be represented by the
lines 204 and 206, respectively, in which case there would be no
need for h. In this ideal case, once the two lines cross and the
RSSI associated with base station 106 exceeds that associated with
the base station 104, handover to base station 106 can occur
smoothly without the ping pong effect alluded to above.
However, because of a somewhat chaotic RF environment, where
shadowing, scattering, etc., cause power fluctuations and a
non-linear pathloss to occur, the actual pathloss associated with
base stations 104 and 106 and experienced by the receiving user
terminal 108 might appear more like the irregular graphical lines
202 and 208, respectively. And as evident from the regions 210 and
212, for example, there may be instances where the RSSI of one base
station exceeds the other, but then shortly later, the situation
may reverse, contributing to a back-and-forth or "ping pong"
handover effect.
To prevent a ping pong handover effect from occurring, some prior
techniques select a relatively large h to use in the handover cost
function C. However, if the hysteresis factor h is fixed at too
large a value, then communication quality may degrade too much
before handover takes place. On the other hand, if the hysteresis
is too small, then there may still exist a potential for the
above-described ping pong effect to occur. To illustrate, the
behavior of the pathloss lines 202 and 208, for example, near the
intersection of their ideal counterparts 204 and 206, respectively,
show that (1) if h is fixed value that is selected too small, it
will likely contribute to an undesired ping pong effect and (2) if
h is fixed value that is selected to be too large, handover may be
delayed until communication quality degrades beyond a desirable
level.
Because the velocity of the user terminal influences signal
strength (e.g., RSSI) calculations when windowing is used, some
handover techniques use a variable measuring window in which to
take signal strength measurements, and then change the window
length relative to the calculated velocity of the user terminal.
Unfortunately, this technique suffers from drawbacks. First,
estimating the velocity of the user terminal is a relatively
difficult task. Second, using a variable window length renders
implementation, for example in a digital signal processor (DSP),
impractical since buffers need to change.
In addition, though the cost function (C) primarily is a function
of signal strength, there may be instances where selecting a base
station whose C is optimized relative to other base stations may
not provide optimum performance. Unfortunately, most prior
techniques have not addressed using other selection criteria for
selecting a base station.
Thus, what is needed is a method and apparatus for facilitating
base station selection, including handover, to overcome the
above-mentioned limitations of prior systems and methods.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for
facilitating initial base station selection and/or handover
(collectively referred to herein as base station selection). In
accordance with one aspect, hysteresis is adaptively determined as
a function of the variance of receive signal strength. In turn, an
adaptive hysteresis factor can be obtained and used for a
subsequent handover decision, for example, based on a cost function
that takes into account the hysteresis. In accordance with another
aspect, base station selection takes into account a set of one or
more selection criteria (e.g., distance and base station load) in
addition to signal strength and hysteresis information to select a
base station.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a typical distributed voice and/or data network
topology, in accordance with the prior art;
FIG. 2 is a graphical representation of the theoretical and
practical signal strengths of two base stations experienced by a
user terminal as the user terminal moves away from one of the base
stations with which it is active communication and toward the other
base station;
FIG. 3 is a flow diagram of a method for adaptively computing a
hysteresis factor for facilitating base station selection,
according to one embodiment of the invention;
FIG. 4 is a flow diagram of a method for computing signal strength
fluctuation for a received signal (e.g., as received by a user
terminal from a transmitting base station), in accordance with one
embodiment of the invention;
FIG. 5 is a flow diagram of a method for computing hysteresis as a
function of the estimate of fluctuation in received signal strength
for two or more base stations, in accordance with one embodiment of
the invention;
FIG. 6 is a flow diagram of a method for selecting a base station
for handover, in accordance with one embodiment of the
invention;
FIG. 7 is a flow diagram of a method for performing initial cell
selection, in accordance with one embodiment of the invention;
FIG. 8A is a block diagram of a user terminal that includes a base
station selection mechanism, in accordance with one embodiment of
the invention;
FIG. 8B is a block diagram of the processing unit of the user
terminal depicted in FIG. 8A, in accordance with one embodiment of
the invention
FIG. 9 is a block diagram of a base station that may be employed in
a wireless communication system employing an embodiment of the
invention.
DETAILED DESCRIPTION
The present invention provides a method and apparatus for
facilitating initial base station selection and/or handover
(collectively referred to herein as base station selection). In
accordance with one aspect, hysteresis is adaptively determined as
a function of the variance of receive signal strength. In turn, an
adaptive hysteresis factor can be obtained and used for a
subsequent handover decision, for example, based on a cost function
that takes into account the hysteresis. In accordance with another
aspect, base station selection takes into account a set of one or
more selection criteria (e.g., distance and base station load) in
addition to signal strength and hysteresis information to select a
base station.
In the following description, numerous specific details are set
forth. However, it is understood that embodiments of the invention
may be practiced without these specific details. In other
instances, well-known structures and techniques have not been shown
in detail. It should be noted that the various elements of the
invention may be implemented in hardware (e.g., circuitry),
software (e.g., machine-executable instructions), or a combination
thereof. Furthermore, multiple general-purpose and/or digital
signal processing (DSP) processors, ASICs or other types of data
processing logic may be employed by a base station or user terminal
of the present invention to carry out one or more methods of the
present invention.
Reference throughout the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. Thus, the
appearance of the phrases "in one embodiment" or "in an embodiment"
in various places throughout the specification are not necessarily
all referring to the same embodiment. Furthermore, the particular
features, structures, or characteristics may be combined in any
suitable manner in one or more embodiments.
Similarly, it should be appreciated that in the foregoing
description of exemplary embodiments of the invention, various
features of the invention are sometimes grouped together in a
single embodiment, figure, or description thereof for the purpose
of streamlining the disclosure and aiding in the understanding of
one or more of the various inventive aspects. This method of
disclosure, however, is not to be interpreted as reflecting an
intention that the claimed invention requires more features than
are expressly recited in each claim. Rather, as the following
claims reflect, inventive aspects lie in less than all features of
a single foregoing disclosed embodiment. Thus, the claims following
the Detailed Description are hereby expressly incorporated into
this Detailed Description, with each claim standing on its own as a
separate embodiment of this invention.
FIG. 3 is a flow diagram of a method for adaptively computing a
hysteresis factor for facilitating base station selection,
according to one embodiment of the invention. In one embodiment of
the invention, the method shown in FIG. 3 is performed exclusively
by the user terminal. It will be appreciated to those skilled in
the art, however, that the invention can be modified to allow the
method to be performed in conjunction with one or more base
station(s) as well.
At block 302, a first value representing the fluctuation in signal
strength (e.g., RSSI) for a first base station is computed.
At block 304, a second value representing the fluctuation in signal
strength for a second base station is computed.
At block 306, the first and second values are combined to obtain a
hysteresis factor associated with the first and second base
stations.
At block 308, the process from blocks 302 through block 306 is
repeated for all "candidate" base stations (i.e., those base
stations whose transmissions the user terminal can "hear" above a
certain threshold) and a handover cost function C for each
candidate base station is computed.
At block 310, handover is performed for any candidate base station
whose cost function is optimized.
In one embodiment of the invention, as become apparent from the
description that follows, the handover is not based solely on the
cost function (which in turn is based on hysteresis and relative
receive signal strengths), but also is based on other base station
selection criteria, e.g., estimated distance to, load of, and
relative difference of cost functions.
Computation of Signal Strength Fluctuation
In the embodiment described above with reference to FIG. 3, the
first and second values computed at blocks 302 and 304,
respectively, represent the signal strength fluctuation for first
and second base stations, respectively. The first base station, for
example, may be a "current" or "active" base station; that is, one
with which the user terminal is actively registered and in
communication, and the second base station may be a candidate base
station. On the other hand, both first and second base stations may
represent candidate base stations, e.g., in the case of initial
base station selection; that is, when the user terminal is not
actively in communication with a particular base station but rather
is searching for an active base station, e.g., upon power-up of the
user terminal.
FIG. 4 is a flow diagram of a method for computing signal strength
fluctuation for a received signal, in accordance with one
embodiment of the invention.
At block 402, the large scale pathloss component of the received
signal is estimated. Referring back to FIG. 2, the large scale
pathloss component for the base station 104's signal as received by
the user terminal 108 is depicted by the line 204, which
essentially changes as the user terminal moves toward or away from
a given base station. In one embodiment, the large scale pathloss
is estimated by averaging the received signal over a relatively
large interval (i.e., one that includes a relatively large number
of samples). However, because averaging introduces a delay in
response (e.g., for performing handover) that is proportional to
the averaging window length, those skilled in the art will
appreciate the trade-off involved in selecting a particular
averaging window for computing this large scale pathloss. In one
embodiment, the invention samples the broadcast transmission of
each candidate base station every 0.5 seconds using a fixed-size
sliding rectangular window spanning 25 seconds (equal to 50
samples).
At block 404 the large scale pathloss component computed at block
402 is removed from a representation (which in one embodiment
involves smoothing) of the small scale received signal strength
from that base station to obtain an intermediate residual signal.
In one embodiment of the invention, sharp changes are removed from
the power envelope using a fixed-size short term averaging window
that spans 5 seconds (or 10 samples), given a base station
broadcast sampling duration of 0.5 seconds. In an alternative
embodiment, no short term averaging is performed.
At block 406, the fluctuation of the residual signal is estimated
by computing the standard deviation .GAMMA. of the residual signal
r in a recursive manner as follows:
For k=1, 2, . . . N
and
where r(k) represents the residual signal; s and l are the short
and long term averaged signals, respectively (though in another
embodiment, there is no short term averaging). L1 and L2 are the
lengths of the long and short averaging windows, respectively;
.alpha. is the memory factor; and .GAMMA..sub.(k,i) is the
(estimated) standard deviation of the averaged signal strength
fluctuation (or "power envelope") of the ith base station at time
instant k; and N is the number of samples used in the above
computation.
In one embodiment, .alpha. is set to 0.1 to provide exponential
weighting that emphasizes relatively recent samples over older
ones. Of course, .alpha. can be set to 1/N, where N is the number
of samples, in which case the memory of the system is infinite and
all samples are given equal weighting in the computed estimate of
the fluctuation. In alternative embodiments, .alpha. can be
selected from a variety of values to suit particular
implementations of the invention.
Computation of Hysteresis
FIG. 5 is a flow diagram of a method for computing hysteresis as a
function of the estimate of fluctuation in received signal strength
for two or more base stations, in accordance with one embodiment of
the invention.
At block 502, the estimates of fluctuation in received signal
strength for a base station pair are summed.
The base station pair, in one embodiment, represents an active base
station and a candidate base station.
In one embodiment of the invention, the receive signal strength
fluctuation associated with a particular base station pair is
computed using equation 3 above. As such, the signal strength
fluctuation is estimated and represented by the standard deviation
of signal strength over one or more fixed-length averaging
windows.
At block 504, the sum obtained at block 502 is scaled by a scaling
factor to obtain the hysteresis for the base station pair. For
example, in one embodiment, the hysteresis h is computed as
follows:
where m is the scaling factor, .GAMMA..sub.A is the estimated
standard deviation of the signal strength fluctuation for the
current (active) base station and .GAMMA..sub.C is the estimated
standard deviation of the signal strength fluctuation for the
candidate base station. The scaling factor m may be obtained
through simulation appropriate for a particular communication
system architecture and network in which the invention is
implemented. In one embodiment of the invention, a scaling factor
of m equal to a value between 1.5 and 2 is used. In alternative
embodiments, particular network simulations may indicate a
different range of values for m. As such, should the invention is
not limited to the range of values for m described with reference
to one particular embodiment of the invention.
Criteria for Base Station Selection
In accordance with one aspect of the invention, received signal
strength and hysteresis (e.g., as provided by the cost function C)
along with other base station selection criteria, such as base
station load and estimated distance thereto, are used to select a
base station.
FIG. 6 is a flow diagram of a method for selecting a base station
for handover, in accordance with one embodiment of the
invention.
At block 600, a (receiving) user terminal measures the RSSI for a
set of candidate base stations (i.e., those whose RSSI are above a
given architecturally-specified threshold) and compiles a list of
candidate base stations based on the measured RSSI's, where the
largest RSSI appears at the top of the list as indicative of the
most likely candidate base station for handover.
At block 602, a handover cost function is computed based on
received signal strength and hysteresis for one or more base
station pairs, wherein one of the base station pairs is the
currently active base station and the other is the most likely
candidate base station (in one embodiment, the one having the
largest RSSI) from the list derived at block 600. The cost
function, in one embodiment of the invention, is represented by the
following equation:
C.sub.i =(S.sub.A -S.sub.i)+h, (1)
where h is the hysteresis (sometimes also referred to as the
hysteresis margin or factor), S.sub.A is the RSSI of the current
(or also sometimes referred to as the active) base station, and
S.sub.i is the RSSI of an ith candidate base station, beginning
with the one having the largest RSSI relative to the other
(remaining) candidate base stations in the list.
At block 604, it is determined whether C.sub.i <0 for any
candidate base station; if not, then at block 606, handover is not
initiated and the current base station remains as such.
If C.sub.i <0 for any candidate base station, then at block 608,
load information for that candidate base station is compared to a
threshold. For example, in one embodiment, if for any candidate
base station load exceeds a maximum load threshold, then at block
610 that candidate base station is eliminated as such from being a
potential handover base station candidate.
In one embodiment, once any candidate base station(s) with load
above a threshold have been eliminated from a given set of
candidate base stations for handover, the remaining candidate base
stations are ordered in accordance with their value for C.sub.i,
such that the base station having associated therewith the most
negative value for C.sub.i appears at the top of the list as the
most likely handover candidate, followed by the next most negative
C value, and so on. In one embodiment, each base station indicates
its load information in a broadcast burst it transmits
periodically.
For example, in one embodiment, at block 612, the base station
having the most negative value for C.sub.i is at the top of the
list of candidate base stations as the most likely candidate base
station BS.sub.i for handover/selection.
At block 614, one or more base station selection criteria of the
most likely candidate base station BS.sub.i is compared with the
other candidate base stations BS.sub.j. In one embodiment, the
comparison is performed in accordance with the ordering of C.sub.i,
beginning with the smallest value(s) for C first. In one
embodiment, the criteria includes load, distance, and if
applicable, also relative signal strength and hysteresis as
provided by the cost function C.
In one embodiment, base stations in a communication network
transmit broadcast messages in a synchronized manner according to a
common timing reference. The common timing reference, in one
embodiment, is a precise reference time derived from Global
Positioning Satellite (GPS) system. As such, the user terminal can
monitor a set of (candidate) base stations to determine a relative
time-of-arrival between the transmissions of those base stations,
and in particular, their broadcast messages. Moreover, the user
terminal obtains from the current base station an indication of the
propagation delay (.DELTA.) based on the current base stations'
measurement of time-of-arrival of uplink signals from the user
terminal relative to the broadcast messages the base station
transmitted. From the propagation delay (.DELTA.), the user
terminal can compute the true distances to both the current base
station and the other (candidate) base stations.
In an alternative embodiments, distance can be computed based on
time stamping of messages, for example, if both the base stations
and user terminals are synchronized to a GPS clock and then measure
time-of-arrival of messages and compare those to the time stamp
included in such messages.
In one embodiment, if:
and
both do not hold true, where l.sub.j is the load of the jth
candidate base station, l.sub.i is the load of BS.sub.i, d.sub.j is
the distance to the jth candidate base station, and d.sub.i is the
distance to BS.sub.i, then BS.sub.i is selected at block 616 as the
current base station.
If, on the other hand, conditions provided by equations (5) and (6)
both hold true, and also
then the jth base station is selected as the current base station;
i.e., handover will take place to the jth base station. Otherwise,
BS.sub.i is selected as the current base station.
It should be appreciated that the particular threshold values
provided in equations (5)-(7) can be adjusted and any combination
of load, distance and/or signal strength may be weighted in a
number of ways in other embodiments of the invention, depending on
design choice and performance requirements. Thus, the invention
should not be limited to the weighting and comparison equations of
the particular embodiment that has been described.
FIG. 7 is a flow diagram of a method for performing initial cell
selection, in accordance with one embodiment of the invention.
Initial cell selection occurs when there is no current base station
with which a user terminal is in active communication of traffic
information; e.g., initial cell selection can occur when a user
terminal powers up to register and "enter" a network and is
searching for an optimum base station to be selected as a
current/active base station. In the embodiment that is described,
the method of initial cell selection is similar to that described
for handover with reference to FIG. 6, except that hysteresis is
set for zero.
At block 702, the base station having the largest receive signal
strength (e.g., as provided by RSSI at the user terminal) is
identified. In one embodiment, all base stations the user terminal
can "hear" (e.g., those having RSSI above a system-defined
threshold at the user terminal) are ordered according to their RSSI
such that the base station with the largest RSSI appears is at the
top of the list of candidate base stations as the most likely base
station, BS.sub.i.
At block 704, any base station(s) with load larger than a threshold
are eliminated from the list of candidate base stations. In one
embodiment, the threshold is set to maximum.
At block 706, the load of the most likely base station, BS.sub.i,
is compared--in descending order of relative RSSI or other signal
strength indicator--to the set of one or more remaining candidate
base stations BS.sub.j using the following condition:
In one embodiment, the comparison is done using base station pairs
in the order of the list, beginning with the largest relative RSSI
values.
If the condition of equation (8) does not hold true for any jth
base station, BS.sub.i is selected at block 708 as the current base
station.
However, if the condition of equation (8) does hold true for any
jth base station, and the condition:
holds true, where S.sub.i is the RSSI of BS.sub.i and S.sub.j RSSI
of BS.sub.j, then the jth base station is selected; otherwise,
BS.sub.i is selected as the current base station.
Hardware Overview
It should be appreciated that the present invention may be useful
in and thus embodied in various wireless systems and architectures,
in which base stations, mobile or stationary user terminals, and
the overall system can have various hardware/software
configurations. Therefore, FIGS. 8-9 depict some, not all,
exemplary architectures for communication devices that may employ
the method and apparatus of the present invention in accordance
with one or more embodiments of the invention.
The present invention relates to wireless communication systems
that may provide fixed-access or mobile-access voice and/or data
communications over the air. Such systems may use spatial division
multiple access (SDMA) technology in combination with multiple
access protocols, such as time division multiple access (TDMA),
frequency division multiple access (FDMA) and code division
multiple access (CDMA). Multiple access can be employed with
frequency division duplex (FDD) or time division duplex (TDD).
However, it should be appreciated that the invention is not limited
to any particular wireless architecture, communication protocol,
system or device.
FIG. 8A is a block diagram of a user terminal that includes a base
station selection mechanism, in accordance with one embodiment of
the invention. As shown, a user terminal 800 includes an antenna
802 (which may include one or more--i.e., an array--of antenna
elements), coupled to a radio unit 804 for transmitting and
receiving RF signals with one or more base stations, such as the
one shown in FIG. 9. The radio unit 804 includes transmitter and
receiver mechanisms, and will typically include one or more power
amplifiers, LNA's, up/down converters, digital-to-analog and
analog-to-digital converters, etc., for receiving signals over the
air and providing them (typically after some down conversion) to a
processing unit 806 coupled thereto, and similarly, up-converting
baseband signals provided by the processing unit 806 and
transmitting them, e.g., to a selected base station.
The processing unit 806, in turn, includes storage areas and
processing circuitry for processing signals received or to be
transmitted by the radio unit 804 via the antenna 802 of the user
terminal 800. As shown, a high level controller 808, shown coupled
to the processing unit 806, can be, in one embodiment of the
invention, further coupled to an external end user data processing
device 810. For example, the user terminal 800, in one embodiment,
represents a wireless modem (e.g., as embodied in a PCMCIA form
factor) that may be coupled to/integrated with the device 810,
which may represent a laptop computer, PDA, gaming or other data
processing device. As such, the high level controller 808 may
receive user selected data to provide to the processing unit 806,
which may in turn process the data (e.g., code it, modulate it,
etc., in accordance with a particular wireless communication
protocol), and provide the processed signals to the radio unit 804
to be transmitted to an active base station.
The processing unit 806, in accordance with one embodiment of the
invention, includes a storage area (not shown) that stores
machine-executable instructions that, when executed, cause the
processing unit 806 to perform one or more of the methods of the
present invention.
Although the user terminal 800 has been described with reference to
a data communication modem, in alternative embodiments of the
invention, the user terminal 800 may represent a voice processing
radio unit, for example for a digital mobile/cellular
telephone.
FIG. 8B is a block diagram of the processing unit of the user
terminal depicted in FIG. 8A, in accordance with one embodiment of
the invention. As shown, the processing unit receives transmitted
signals from one or more base stations which are provided as input
to a received signal strength measurement (RSSI) unit 820. In one
embodiment, the signal received from a particular base station is a
broadcast message burst that includes (1) an indication of the load
(e.g., the number user terminals actively communicating with that
base station or alternatively, an indication of whether the load is
or is not above a threshold); and (2) an indication of propagation
delay, from which distance to the base station can be derived by a
distance calculation unit 826. The output of the distance
calculation unit 826--namely, the estimated distance between the
user terminal and the base station--is provided to a base station
selection unit 824, as is the load information and the output of
the RSSI unit 820.
The output of the RSSI unit 820 is also provided to a hysteresis
calculation unit 822. The hysteresis calculation unit 822, in one
embodiment, adaptively calculates hysteresis for use in a handover
cost function, in accordance with a method of the invention as
described herein. In an alternative embodiment, however, the
hysteresis calculation unit may provide a hysteresis factor using
one or a combination of other methods.
The base station selection unit 824, using the input hysteresis
factor (if any), an indication of received signal strength as
provided by the RSSI unit 820, as well as the load information and
distance information, selects a base station, either initially or
for handover, in accordance with a described method of the present
invention.
FIG. 9 is a block diagram of a base station that may be employed in
a wireless communication system employing an embodiment of the
invention. As shown, a system 920, which may be part of a base
station, in one embodiment, includes an antenna array 922, which in
turn includes a number of antenna elements. The antenna array 922
is utilized for transmitting downlinks signals to one or more
remote user terminals and for receiving uplinks signals from the
one or more remote user terminals. In one embodiment, the antenna
array 922 also transmits a broadcast message that indicates
information about the base station, including, for example, an
indication of reference time of transmission of the broadcast
message (from which a receiving user terminal can estimate distance
to the base station), an indication of the load of the base
station, and in one embodiment, an indication of the transmit power
the base station uses to transmit the message.
Of course, the system 920 may communicate with several remote user
terminals, and as such, may process a number of signals each
associated with a remote user terminal or other signal source.
Furthermore, the system 920 may be employed in each of several base
stations in a wireless communication network, where each base
station uses a given set of channels to communicate with remote
user terminal units within a given geographic region, e.g., a cell.
Such remote user terminals may be stationary or mobile, and may
communicate voice and/or data with the system 920 using PPP, TC/IP
and/or other data or voice protocols. In one embodiment, each such
remote user terminal is coupled to an external data processing
device (e.g., a laptop computer, a PDA, a gaming device or other
computing device) using an Ethernet or PPP-over-Ethernet (PPPOE)
connection to allow such device to exchange data with the system
920 vis-a-vis a wireless communication link established between the
user terminal and the system 920.
As shown in FIG. 9, each antenna element of the antenna array 922
is coupled to a power amplifier (PA) and low-noise amplifier (LNA)
924. The PA/LNA 924 of each antenna element amplifies the received
(uplink) and/or transmitted (downlink) signal(s). As shown, each
PA/LNA 924 is coupled to a down-converter 926 and an up-converter
928. The down-converter 926 converts the "raw" signal received by
the antenna array 922 on a carrier frequency into a receive (Rx)
baseband signal, which is provided to a baseband processor (also
referred to as a modem board) 930. The up-converter 928,
conversely, converts a transmit (Tx) baseband signal provided by
the baseband processor 930 into a carrier frequency transmit
signal, which is provided to the PA/LNA 924 to be transmitted
(e.g., to a remote user terminal). Although not shown,
analog-to-digital conversion (ADC) and digital-to-analog (DAC)
circuitry may be coupled between the down-converter 926 and the
baseband processor 930 and between the up-converter 928 and the
baseband processor 930, respectively.
The baseband processor 930 typically includes hardware (e.g.,
circuitry) and/or software (e.g., machine-executable
code/instructions stored on a data storage medium/device) to
facilitate processing of received (uplink) and transmitted
(downlink) signals. In accordance with the embodiment of the
invention shown in FIG. 9, the baseband processor 930 includes at
least one narrow-band filter 936 filter to filter received signals
either in analog or digital form. The filtered signal from the
narrow-band filter 936, in turn, is provided to a spatial processor
938.
The spatial processor 938 typically includes at least one general
purpose processor and/or digital signal processor (DSP) to
facilitate spatial processing. In one embodiment, the spatial
processor 938, based on the spatial or spatio-temporal
characteristic(s) (also known as a "spatial signature") of one or
more uplink signals, is able to transmit and receive signals
between one or more remote user terminals in a spatially selective
manner. Accordingly, in one embodiment where spatial channels and
SDMA is utilized, two or more remote user terminals may
simultaneously receive and/or transmit on the same channel (e.g.,
carrier frequency and/or time slot and/or code) but may be
distinguishable by the system 920 based on their unique spatial or
spatio-temporal characteristic(s). However, in alternative
embodiments of the invention, spatial channels may not be employed.
One example of a spatial characteristic is direction of arrival
(DOA) or angle of arrival (AOA). Other types of spatial
characteristics known in the art of adaptive arrays may be employed
in conjunction with the present invention.
In general, the antenna array 922 facilitates transfer of signals
between the system 920 and a desired remote user terminal and/or
one or more other devices (e.g., a plurality of remote user
terminals, other base stations in a wireless communication network,
a satellite communication network, etc.). For example, the antenna
array may transmit downlink signals to the desired remote user
terminal, and receive uplink signals from the remote user terminal.
Such transmission and reception may occur in the same frequency
channel but at different times (e.g., in a TDD system) or may occur
at different frequencies (e.g., in an FDD) system. The processor
938 determines the spatial characteristic(s) of the uplink signal
from the desired remote user terminal, also referred to herein as a
primary remote user terminal, as well as the spatial
characteristic(s) of one or more other non-primary remote user
terminals. Based on such characteristics, the system 920 determines
a downlink beamforming strategy to enhance its transmission gain at
the location of the desired remote user terminal, while relatively
minimizing its transmission gain (i.e., providing a "null" or
interference mitigated region) at the location of the non-primary
remote user terminal(s). Similarly, the system 920, based on the
spatial characteristics, may perform uplink beamforming to enhance
its reception gain from the location of the primary remote user
terminal, while minimizing its reception gain from the location(s)
of one or more non-primary remote user terminals.
In one embodiment of the invention, the system 920 supports spatial
channels, such that two or more remote user terminals in
communication with the system 920 may simultaneously employ the
same conventional frequency and/or time channel. In alternative
embodiments, however, spatial channels may not be supported or
utilized or may be utilized only when one or more conditions are
met.
As shown in FIG. 9, the spatial processor 938 is further coupled to
a demodulator and error control unit 940, which receives an
"extracted" or "desired" signal or set of signals from the spatial
processor 938, and outputs the extracted signal to a network
processor 932. The unit 940 may perform error correction, provide
packet overhead, and/or perform other processing before outputting
the uplink information in the form of digital data to the network
processor 932.
The network processor 932, which may or may not constitute part of
the system 920, facilitates the transfer of information between the
system 920 and an external network 934, which, for example, may
represent the Internet, in which case the system 920 may be coupled
(through wireless and/or wired links) to an Internet Service
Provider (ISP). Such information may include voice and/or data and
may be transferred in a packet-switched or circuit-switched manner.
For example, in one embodiment, a remote user terminal may include
a cellular telephone, two-way pager, PDA with wireless
communication capability, a wireless modem that may be interfaced
to a data processing device, such as a laptop computer, PDA, gaming
device or other computing device, or other communication device to
facilitate routing voice and/or data signals between the remote
user terminal(s) and the network 934, which in this example may
include the public switched telephone network (PSTN), the Internet,
and/or other voice and/or data network. Thus, the remote user
terminal may include or be interfaced with a computing device
(e.g., a portable digital assistant, a laptop/notebook computer, a
computing cellular telephone handset, etc.), along with a
Web-browser, in which case the network 934 may represent the
Internet and the network interface processor may facilitate
communication between the remote user terminal (via the system 920)
and one or more servers or other data processing systems coupled to
the Internet. As such, voice and/or data (e.g., video, audio,
graphics, text, etc.) may be transferred between the system 20 (and
one or several remote user terminals in communication therewith)
and an external network 934.
The term "base station" as used herein denotes a voice or data
access point of wireless communication system that serves user
terminals in a given geographical area. Such a base station, for
example, may be an access point of a cellular-type voice (and/or
data) communication system. Alternatively, the base station may
represent an access point of an IEEE 802.X standard-based data
communication network (e.g., 802.11, 802.16, 802.20, etc.). It
should therefore be appreciated that although an exemplary
architecture of a base station that may be used in a communication
system embodying the present invention has been described, the
invention is not necessarily limited to any particular base station
or communication system architecture.
The elements shown in FIGS. 8-9 may be implemented by hardware,
software or combination thereof, as will be apparent to those
skilled in the art. For example, the elements in FIGS. 8-9 may be
embodied in machine-executable instructions, which may be used to
cause a general-purpose or special-purpose processor or logic
circuits programmed with the instructions, to perform the method(s)
of the present invention. Alternatively, the elements in FIGS. 8-9
may be implemented by logic or analog circuits to perform the
method(s) of the present invention or with a combination of such
circuits with machine-executable instructions (i.e., software).
The present invention in one embodiment is provided as a computer
program product which may include a machine-readable medium having
stored thereon instructions which may be used to program a computer
(or other electronic devices) to perform a process according to the
present invention. The machine-readable medium may include, but is
not limited to, floppy diskettes, optical disks, CD-ROMs, and
magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or
optical cards, flash memory, or other type of media or
machine-readable medium suitable for storing electronic
instructions. Moreover, the present invention may also be
downloaded as a computer program product, wherein the program may
be transferred from a remote computer to a requesting computer by
way of data signals embodied in a carrier wave or other propagation
medium via a communication link (e.g., a modem or network
connection).
Importantly, while the present invention has been described in the
context of a wireless internet data system for portable handsets or
other user terminal devices (e.g., wireless modems that may be
interfaced with various portable data processing devices), it can
be applied to a wide variety of different wireless systems in which
data is exchanged. Such systems include voice, video, music,
broadcast and other types of data systems without external
connections. The present invention can be applied to fixed user
terminals as well as to low and high mobility terminals. Many of
the methods are described herein in a basic form but steps can be
added to or deleted from any of the methods and information can be
added or subtracted from any of the described messages without
departing from the basic scope of the present invention. It will be
apparent to those skilled in the art that many further
modifications and adaptations can be made. The particular
embodiments are not provided to limit the invention but to
illustrate it. The scope of the present invention is not to be
determined by the specific examples provided above but only by the
claims below.
Although the invention has been described with reference to several
embodiments, it will be appreciated that various alterations and
modifications may be possible without departing from the spirit and
scope of the invention, which is best understood by the claims that
follow.
* * * * *